As a vacuum carburizing method of performing a carburizing process on steel parts for automobile such as gears, bearings, fuel injection nozzles and constant velocity joints, for example, a method of using ethylene gas as a carburizing gas to perform the process under reduced pressures of 1 to 10 kPa in a vacuum heat treating furnace has been known (see Japanese Unexamined Patent Publication No. 11-315363).
In the conventional method, however, when the vacuum carburization is performed while disposing a basket which carries a number of objects to be treated (workpieces) in an effective heating space where uniformity of temperature is ensured in the vacuum heat treating furnace, there arises a problem that unevenness of carburization occurs in the workpieces depending on the carried position in the basket, and variation occurs in carburization quality such as effective case depth (carburization depth) and surface carbon concentration among workpieces in different carried positions.
Thus, as a vacuum carburizing method which solves the above described problem, the present applicant has previously proposed a method of using a mixed gas of ethylene gas and hydrogen gas as a carburizing gas (see Japanese Unexamined Patent Publication No. 2001-262313).
In the vacuum carburizing method previously proposed by the present applicant, even when carburization is performed while disposing a number of workpieces in an effective space where uniformity of temperature is ensured in the vacuum heat treating furnace, it is possible to prevent unevenness of carburization from occurring in all workpieces, so that carburization quality of all the workpieces can be made uniform.
In the method disclosed in Japanese Unexamined Patent Publication No. 2001-262313, however, low grade steels, for example, steels containing a high proportion of impurities such as MnS, low alloy steels, low carbon steels and the like would not be hardened by hardening by means of quenching after carburization, which leads to a problem that sufficient surface hardness and effective case depth cannot be obtained. In addition, if ammonia gas is introduced into the vacuum heat treating furnace together with ethylene gas and hydrogen gas for the purpose of obtaining a surface hardened case in low grade steels, retained austenite increases or cementite becomes likely to precipitate. In particular, when ammonia gas is introduced together with ethylene gas and hydrogen gas, it is necessary to elongate the process time in order to increase the effective case depth, which leads to a problem of increased cost. In addition, in the case of case-hardened steels, a plenty of cementite will precipitate, which leads to a problem that the steel becomes brittle and cracking becomes likely to occur.
The present invention has been made in order to solve the above described problems, and it is an object of the present invention to provide a vacuum carbonitriding method capable of obtaining necessary heat treatment quality such as surface hardness, effective case depth, toughness and the like in short time and with reproducibility even in a case of a workpiece made of low-grade steel or case-hardened steel.